Conventional methods and apparatus are described in connection with electron-beam microlithography as an example of charged-particle-beam (CPB) microlithography. Electron-beam microlithography provides higher resolution than optical microlithography (e.g., microlithography using ultraviolet light), but exhibits much lower throughput (number of wafers that can be processed per unit time) than optical microlithography. A variety of technology development efforts have been directed to solving the throughput problem, however, and certain techniques (such as "cell projection" techniques) offering advantages in this regard are now becoming practical. Unfortunately, the cell projection technique has limited applicability because it is utilized mainly for manufacturing chips having large areas in which a basic circuit configuration (such as a memory cell) is repeated a large number of times.
Other current techniques, however, still suffer from extremely low throughput. For example, in the "partial pattern block exposure" technique, repeated exposures of each of a limited number of different pattern portions (each representing a respective recurring portion of the overall device pattern) are performed. The throughput realized with this technique is at least an order of magnitude too low for practical production-level semiconductor wafer-exposure applications.
Another conventional technique is electron-beam "reduction-transfer." One type of reduction transfer, termed full-field reduction transfer, offers prospects of a vastly improved throughput over the partial pattern block exposure technique. In full-field reduction transfer, a reticle defining an entire chip pattern (i.e., the entire pattern for a layer of a semiconductor chip as formed on a wafer) is illuminated with an electron beam. A "reduced" (i.e., demagnified) image of the pattern residing within the field of illumination is projected by a projection lens, situated downstream of the reticle, onto the wafer. As can be surmised, the electron optics used with such a system must accommodate a very large field. Unfortunately, however, it has been impossible to date to provide an electron-optical system having a sufficiently large field and that exhibits satisfactory aberration control, especially in peripheral regions of the field. Thus, it has not been possible to achieve the desired image quality on the wafer over a field large enough to encompass the entire pattern in one shot.
Accordingly, another reduction transfer technique (termed "divided" or "partitioned" reduction transfer) has been proposed in which the total area of the pattern, as formed on the reticle, is partitioned into many small portions (termed "exposure units" or "subfields"). (The reticle is thus termed a "divided" or "segmented" reticle.) The individual exposure units are sequentially illuminated and transferred. The corresponding images of the individual exposure units on the wafer are located so as to be "stitched together" in a prescribed arrangement on the wafer to complete the transfer of the entire pattern. In this regard, reference is made to U.S. Pat. No. 5,260,151, incorporated herein by reference, and to Japanese published patent application no. Hei 8-186070 for examples of divided reduction transfer.
Two main types of reticles, stencil reticles and membrane reticles, are used in divided reduction transfer. An especially useful reticle is the so-called "scattering-stencil" reticle, that is fabricated by micromachining a pattern of feature-defining voids (through-holes) in a "reticle membrane" (typically a silicon film approximately 1 to 5 .mu.m thick). Electrons of an "illumination beam" (electron beam used to individually illuminate the exposure units) pass through the voids in the membrane of a scattering-stencil reticle without being scattered. Electrons of the illumination beam that strike the membrane itself usually pass through the membrane but are highly scattered during passage. To prevent these scattered electrons from reaching the wafer, a "contrast aperture" is placed at or near the beam-convergence plane of a projection lens located between the reticle and the wafer (the beam-convergence plane of the projection lens is the Fourier plane for the reticle plane). Electrons divergently scattered by passage through the reticle membrane are absorbed by the contrast aperture, while electrons that pass through the contrast aperture are imaged on the wafer. Thus, contrast in the projected image is achieved.
During fabrication a reticle encounters various delineation and etching steps. These steps tend to distort the reticle. In addition, the reticle can be distorted by electrostatic chucking, for instance, whenever the reticle is mounted on a reticle stage of the microlithographic projection-exposure apparatus. An exposure performed with a distorted reticle will produce a distorted pattern on the wafer, thus degrading accuracy of overlay registration and stitching.